The present invention relates in general to circuit designs for boilers, and in particular to a new and useful circulation system for the heated tubes for absorbing heat in a furnace.
Furnace circuits that receive heat, and fluid flow from a low elevation to a high elevation are referred to as "upflowing circuits" and circuits that receive heat, and fluid flow from a high elevation to a low elevation are referred to as "downflowing circuits". A circuit is made up of a tube or a group of tubes that originates at a common point such as a header or a drum, and terminates at a common point that could also be either a header or a drum.
In most natural circulation boiler designs, the heated tubes that compose the evaporative portion of the design are configured for upflow of the fluid, the exception being the heated downcomer tubes of the generating bank(s) on multi-drum boilers. In this type of boiler the heated downcomer tubes provide the total circulation flow for the furnace and the evaporative generating bank riser tubes.
In FIG. 1 the circulation concept of a typical industrial boiler is shown. In this concept, subcooled water from a steam drum 10 enters the heated evaporative generating bank downcomer tubes 12 in the exhaust passage 20 of the furnace. The water travels down the tubes of this bank and is collected in the lower drum 14 of the bank. The enthalpy of the water that exits into the lower drum 14 has increased due to the heat that was absorbed by each tube 12 in the bank. The water in the lower drum 14 could either be subcooled or saturated, depending upon the amount of heat absorbed. The mixture that leaves the lower drum 14 will either travel up the evaporative generating bank riser tubes 16 or down the large tubes or pipes 18 called downcomers. The liquid that travels up the riser tubes 16 absorbs heat and exits into the steam drum 10. The liquid that travels down the downcomers 18 reaches the furnace inlet headers 19 either through direct connection of the downcomer 18 to the inlet header 19 or through intermediate supply tubes 22 that feed the liquid to specific inlet headers. The liquid that enters an inlet header 19 is distributed to the furnace tubes 24 that are connected to the inlet header 19. The tubes of the furnace are heated by the burning of the fuel in the combustion chamber 30 of the furnace. The adsorption of heat by the furnace tubes 24 causes the liquid in the tubes 24 to boil resulting in a two-phase mixture of water and steam. The two-phase mixture in the tubes 24 reaches the steam drum 10 either through direct connection of the tubes 24 with the steam drum 10 or through intermediate riser tubes 26 that transmit the two-phase mixture into steam and water. Subcooled feedwater that is discharged from the feedpipe (not shown) in the steam drum 10 and the saturated liquid that is discharged from the separation equipment are mixed together to yield a subcooled liquid that exits the stream drum 10 by way of the downcomer tubes 12, thus completing the circulation flow loop for this concept.
For evaporative boiler generating bank modules and selected furnace and convection pass wall enclosures subject to the flow of the combustion gases, a threshold heat input is required to adequately circulate the fluid in all the tubes in the module and in the convection pass wall enclosure circuits in upflow while avoiding flow instability. As used herein, convection pass wall enclosure refers to the various structures formed by tubes conveying a fluid and which pick up heat primarily via convective heat transfer between the gas stream and the tubes, and which serve to at least partially define the exhaust passage or passages of the boiler. For certain designs, it is impossible to circulate all the tubes in the evaporative modules or convection pass wall enclosures in upflow without changing to a more expensive module or wall enclosure geometry (thicker tubes for increasing tube flow velocity, taller module or wall enclosure height, reduced system flow resistance through the addition of circulation system pressure part connections, etc.).
In most natural circulation designs, as an alternative to more expensive evaporative modules, economizer surface may be added to absorb the additional heat required to meet the desired boiler outlet gas temperature. When economizer surface is added, the economizer outlet water temperature increases. The economizer outlet water is fed to the steam drum. If the economizer outlet water temperature reaches the saturation temperature of the liquid in the steam drum, then the circulation system of the boiler will receive no subcooling from the feedwater that enters the drum. The subcooling that the feedwater system delivers to the steam drum provides a portion of the `pumping` head that is needed to make the circulation system operate. When the subcooling is not available due to a saturated or near saturated economixer outlet water temperature, achieving adequate boiler circulation and desired boiler efficiency (outlet gas temperature) will require increased boiler cost since it will be necessary to either reduce the economizer outlet temperature (e.g. by using water coil air heaters) or add circulation system pressure part connections, with their additional increased cost.